Automatic transmission systems have hydraulic circuits which includes a hydraulic pump, a torque converter, and clutch packs. An input shaft transfers power from the torque converter to the planetary gear set. To cool the clutch packs, the input shaft has a bore that allows automatic transmission fluid to flow between the torque converter and the clutch packs.
When the vehicle engine is off and the pump is not running, automatic transmission fluid generally flows out of the torque converter and thus, at initial engine startup, the torque converter has insufficient automatic transmission fluid to operate properly.
In order to solve this startup problem and prevent the flow of automatic transmission fluid from the torque converter when the engine is off, a single-stage check valve is placed in the bore of the input shaft between the torque converter and the clutch plates. The single-stage check valve closes the hydraulic circuit during periods when the pump is off to prevent the flow of automatic transmission fluid from the torque converter. Typically, these single-stage check valves open at a pressure of about 2-4 psi and allow for a flow of 6 liters per minute at idle and up to 18 to 20 liters per minute at maximum flow rate.
One of the problems with the single-stage check valve is that it opens too far too soon. This causes the transmission's open clutch pack to flood with automatic transmission fluid which in turn creates frictional drag. The frictional drag results in frictional losses and increase in fuel consumption.